In recent years there has been a growing interest in increased automation in the control of the drive train of motor vehicles, and most especially in control of the drive train of large trucks. The use of automatic transmissions in passenger automobiles and light trucks is well known. The typical automatic transmission in such a vehicle employs a fluid torque converter and hydraulically actuated gears for selecting the final drive ratio between the engine shaft and the drive wheels. This gear selection is based upon engine speed, vehicle speed and the like. It is well known that such automatic transmissions reduce the effectiveness of the transmission of power from the engine to the drive shaft, with the consummate reduction in fuel economy and power as compared with the skilled operation of a manual transmission. Such hydraulic automatic transmissions have not achieved wide spread use in large motor trucks because of the reduction in efficiency of the operation of the vehicle and added cost.
One of the reasons for the loss of efficiency when employing a hydraulic automatic transmission is loss occurring in the fluid torque converter. A typical fluid torque converter exhibits slippage and consequent loss of torque and power in all modes. It is known in the art to provide lockup torque converters that provide a direct link between the input shaft and the output shaft of the transmission above certain engine speeds. This technique provides adequate torque transfer efficiency when engaged, however, this technique provides no gain in efficiency at lower speeds.
It has been proposed to eliminate the inefficiencies inherent in a hydraulic torque converter by substitution of an automatically actuated friction clutch. This substitution introduces certain problems not exhibited in the use of the hydraulic torque converters. U.S. patent application Ser. No. 772,204, filed Oct. 7, 1991 and entitled "Closed Loop Launch and Creep Control for Automatic Clutch" teaches the minimization or elimination of torsional oscillations due to compliance in the driveline during clutch engagement by controlling the clutch actuation to effect a smooth engagement. U.S. patent application Ser. No. 772,778, filed Oct. 7, 1991 and entitled "Closed Loop Launch and Creep Control for Automatic Clutch with Robust Algorithm" addresses the same problem and includes a prefilter to shape the system transient response and reduces the need for detailed particularization for individual vehicles or vehicle models. Each of those disclosures, Ser. No. 772,204 and Ser. No. 772,778, hereby incorporated by reference, is assigned to the assignee of this invention and includes the generation of a clutch control signal which is dependent on the selection of a creep or launch mode.
Still another problem relates to the slow response to the clutch actuation algorithm. Friction clutches exhibit considerable time delay between the point when throttle actuation calls for clutch application and the point that the clutch develops torque capacity equivalent to the engine torque, so that in the meantime the engine is not restrained and its speed may become excessive. Here a method is proposed which approximates the action of a human operator, taking into account the condition of clutch actuation to coordinate the engine speed with the clutch engagement.